Millions worldwide suffer from the ocular pain and diminished tearing of dry eye disease. Although great advances have been made in many aspects of dry eye research, one area of studies that of corneal nerves has lagged behind. Tear hyperosmolarity is ubiquitously present in dry eye disease and dysfunction of the corneal nerve contributes significantly to the initiation and/or perpetuation of dry eye disease. However, these two subjects have never been studied together. Thus, the proposed research will investigate the effect of tear hyperosmolarity on the corneal nerves responsible for tearing and pain sensation. We recently discovered in rats that special types of corneal nerves displayed many of the response characteristics that are consistent with these nerves serving the functions of tearing and pain sensation. We also discovered that these corneal nerves altered their sensitivities when the eyes were treated with the amounts of hyperosmolar tears found in dry eye patients. Thus, after the corneal nerves were treated with hyperosmolar tears, the nerves began to be excited by a gentle cooling (~1oC) of the cornea, indicating that the normally non-painful stimulus (mild cooling) now activated these neurons presumably giving rise to ocular pain. On the other hand, after the treatment with hyperosmolar tears these neurons began to be less excited by the stimulus thought to be critical for tearing (i.e., drying of the cornea). Therefore, the effects of tear hyperosmolarity on these nerves could explain the exaggerated eye pain and the diminished tearing that DE patients report. We propose to study the following questions in more detail: 1) what is the minimum amount of tear hyperosmolarity necessary to change the neural sensitivity; 2) how long does the tear hyperosmolarity have to remain on the cornea for the nerves to change sensitivity, and does the longer tear hyperosmolarity necessarily lead to more or less activation of these neurons (i.e., more pain and fewer tears?); 3) does the hyperosmolar tear-induced change become permanent, and if so, when? We have also discovered that these neurons show abnormal activity under extended hyperosmolar tear conditions (2 days). What are the mechanisms responsible for these sudden changes in activity? The answers to these questions could identify hyperosmolar tears as being a crucial determinant of the ocular pain and diminished tears found in DE patients. A variety of drugs such as antagonists for inflammatory mediators and membrane ion channels in corneal nerves and epithelia will be tested to block these sensitivity changes and the abnormal activities seen in these nerves. These pharmacological determinations of the changes in corneal nerve functions may lead to better understanding of the pathogenesis of dry eye diseases and offer new avenues of treatment for one of the most common and yet challenging diseases in our society.